1. Field of the Invention
The present invention relates to a voltage generating circuit for generating voltages of various voltage levels for driving liquid crystal display devices used in a liquid crystal display television receiver and the like.
2. Description of the Prior Art
Voltages of various voltage levels are needed to drive a liquid crystal display device used in a liquid crystal display television receiver and the like. An example of a voltage generating circuit for generating voltages of various voltage levels is disclosed in U.S. Pat. No. 4,769,639, which has a construction as shown in FIG. 1.
In FIG. 1, a reference E denotes a DC power source. A terminal (-) of the DC power source E is directly grounded while its terminal (+) is grounded through a series connection of a variable resistor R1 and a resistor R2. A divided voltage at a junction point of the variable resistor R1 and the resistor R2 is supplied to a terminal (+) of a differential amplifier circuit A1. A terminal (-) of the differential amplifier A1 is grounded through a resistor Ra. The differential amplifier A1 serves as an amplifier of a non inversion type with a feed back resistor Rb connected between its output terminal and the terminal (-), the voltage gain of which is given by the feed back resistance Rb and the resistance Ra. An output signal of the differential amplifier A1 is used as a driving voltage V1 for driving liquid crystals and is supplied to a terminal (-) of a differential amplifier A2 through a resistor Rc. The differential amplifier A2 has a terminal (+) which is grounded directly and the terminal (-) which is connected through a feed back resistor Rd to its output terminal. The differential amplifier A2 is an amplifier of an inversion type, too and its voltage gain is determined by the resistances Rc, Rd. The output of the differential amplifier A2 is used as a driving voltage V5 for driving liquid crystals.
Meanwhile, the output terminal of the differential amplifier A1 is grounded through a series connection of a resistor R3 and a resistor R4. A voltage at a junction point of the resistor R3 and the resistor R4 is supplied to a terminal (+) of a differential amplifier A3. The differential amplifier A3 is a voltage follower of a non inversion type with its output signal applied to its terminal (-) as a feed back voltage. The output of the differential amplifier A3 is used as a driving voltage V2 for driving liquid crystals.
The output signal of the differential amplifier A3 is supplied to a terminal (-) of a differential amplifier A4 through a resistor Re. The differential amplifier A4 has a terminal (+) grounded directly and the terminal (-) connected through a feed back resistor Rf to its output terminal. The differential amplifier A4 is an amplifier of an inversion type, too and its voltage gain is determined by the resistances Re, Rf. The output of the differential amplifier A4 is used as a driving voltage V4 for driving liquid crystals.
To drive the above circuit, a positive power source E+ and a negative power source E- (not shown) are supplied to the differential amplifiers A1, A2, A3 and A4 under conditions that " the positive power source E+ is higher than the output voltage V1 and the negative power source E- is less than the output voltage V5.
Now, operation of the above prior art circuit will be described. The DC power voltage E is divided by the variable resistor R1 and the resister R2 and the divided voltage is supplied to the differential amplifier A1 as a reference voltage, which can be set to any voltage level by changing the variable resistance R1. The differential amplifier A1 amplifies the above reference voltage in the same polarity with the voltage gain determined by the resistances Ra and Rb, and supplies it to the next differential amplifier A2 through the resistor Rc. The amplified voltage is used as the driving voltage V1 for driving liquid crystals. The differential amplifier A2 amplifies the input voltage in the opposite polarity with the voltage gain determined by the resistances Rc and Rd. The amplified voltage is used as the driving voltage V5 for driving liquid crystals. Accordingly, the driving voltage V5 has the same voltage level as the driving voltage V1 with respect to the earth potential (=V3) but in the opposite polarity.
The output voltage of the differential amplifier A1 is divided by the resistors R3 and R4, and the divided voltage is supplied to the differential amplifier A3. The differential amplifier A3 outputs a voltage as the driving voltage V2 in the same polarity as that of the input voltage and supplies it through the resistor Re to the differential amplifier A4. The differential amplifier A4 inverts the input voltage in polarity and amplifies it with the voltage gain determined based on the resistances Re and Rf, and then outputs the amplified voltage as the driving voltage V4. The driving voltage V4 for driving liquid crystals has the same voltage level as the driving voltage V2 with respect to the earth potential (=V3) but in the opposite polarity.
The driving voltages V1, V2, V4 and V5 are obtained as described above, but their voltage levels have to be adjusted so as to meet conditions that "V1=-V5" and "V2 =-V4", as shown in FIG. 2. The differential amplifiers A2, A4 are amplifiers of a inversion type with unit voltage gain. Therefor, the resistances Rc, Rd, Re and Rf are set so as to meet conditions that "Rc=Rd" and "Re=Rf". More specifically, the amplifier A2 outputs the driving voltage V5 having the same voltage level as the driving voltage V1 but in the opposite polarity and the amplifier A4 outputs the driving voltage V4 having the same voltage level as the driving voltage V2 in the opposite polarity. Since the driving voltage V1 is applied to the differential amplifier A2" and further a voltage obtained by multiplying the driving voltage V1 by ratio of resistance R3 to total resistance R3+R4 is applied to the differential amplifier V2, all the driving voltages V2, V4 and V5 are dependent on the driving voltage V1.
The driving voltage V1 varies in accordance with variation in the reference voltage. All the driving voltages V1, V2, V4 and V5 can be simultaneously changed by changing the reference voltage by means of the resistor R1. In the circuit as described above, driving voltages V1, V2, V4 and V5 are previously set to the best ratio and these voltages are adjusted by changing the variable resistance R1 with the above ratio maintained unchanged.
The above voltage generating circuit, however, has the following drawbacks: (a) the amplifier needs both a positive power source and a negative power source; (b) four units of differential amplifiers are needed, resulting in a complicated circuit configuration, and (c) five connecting lines are needed to connect the voltage generating circuits to driving circuits for driving liquid crystals. Since the connecting lines for the driving voltages V1, V2, V3, V4 and V5 are needed, it is very hard to make the whole liquid crystal display device compact in size which includes a power source circuit section, a voltage generating circuit section for generating liquid-crystal driving voltages, liquid-crystal driving circuit section and a control circuit section.
The present invention has been made to remove the above drawbacks and has an object to provide a voltage generating circuit for generating a liquid-crystal driving voltage, which circuit needs only one of a positive power source circuit and a negative power source circuit to generate voltages of various levels such as V1, V2, V3, V4 and V5 for driving liquid crystals and can be realized with a simple circuit construction, resulting in compact in size.